The present invention relates to a method for enhancing the control response of a drive train with backlash and/or elasticity, and more particular a drive train of this type employed in a machine tool and production machine. In the application, the term “production machine” is used here in a generic sense and includes also robots which generally follow the concepts outlined here.
Machine tools and production machines, including robots, frequently include gears and/or couplings inserted between a drive motor and the driven load. The gears and couplings typically have play, which produce an elastic behavior or backlash between the drive and the load. Backlash has the undesirable effect on a control system that the force that must be produced by the motor is almost zero up to a certain point or displacement distance, and that from that point on the motion can only be sustained by applying a significantly greater force than the initial force.
Backlash is produced, for example in gears employing toothed wheels, wherein a tooth of a first gear wheel is located exactly between two teeth of a second gear wheel. When the drive motor connected to, for example, the first gear wheel starts up, only a very small force is required for moving the first gear wheel. However, when a corresponding tooth of the first gear wheel contacts a tooth of the second gear wheel connected with load, the motor requires a significantly greater force for continuing the travel motion.
The mechanical components used in gears and couplings also have elasticity, which do not permit a direct proportional relationship between motor speed and the speed of the load, in particular for a dynamic motion.
Accordingly, backlash and/or elasticity necessitate a nonlinear characteristic of the controlled system.
In conventional applications, the controlled valuable supplied to a speed controller that controls the motor speed is either an actual motor speed that is measured directly on the motor or a load speed that is measured proximate to the load. Both methods have certain disadvantages. Due to the aforementioned backlash and/or elasticity, the motor speed is not identical to the load speed, in particular in dynamic processes.
If the motor speed is used as the only controlled valuable, then the control amplification of the controller can disadvantageously not be set to reflect the entire mass of the controlled system, which is composed of the mass of the load, the mass of the motor and the mass of the gears, because the aforementioned individual masses cannot be viewed as a single rigid total mass due to the existing backlash and/or elasticity. Since the control amplification of the controller is typically set to optimize the damping behavior of the system, the control amplification must be kept low, which adversely affects the control response.
If the load speed is used as the only controlled variable, then the control circuit tends to exhibit instabilities, because the electromechanical components have a rather small mechanical damping during torque reversal and the backlash and/or elasticity also contribute a small torque. In this case, the control amplification of the controller can be adapted to the load, which permits high control amplification with an excellent control response. A system control based on controlling the load speed, however, can only be implemented when the mechanical components of the drive system are very stiff. Accordingly, such control methodology is typically not implemented in a drive system where a gear is inserted between the motor and the load, since such system lacks stiffness and, more particularly, the gears exhibit backlash or play. Such control methodology is typically employed in drive systems where the motor, for example a torque motor, is coupled directly to the machine shaft. However, a favorable control response can only be attained at the expense of a relatively complex and expensive drive system.
A methodology of the aforedescribed type is depicted, for example, in the NC/CNC Handbook 95/96, by Hans B. Kief, published by Carl Hanser Verlag Munich Vienna, 1995, page 189, FIG. 1a, which describes the use of a combination of two measurement signals in a control circuit.
It would therefore be desirable and advantageous to provide an improved method for enhancing the control response of a drive train, which obviates prior art shortcomings and is able to specifically control drive trains having inherent backlash and/or elasticity, in particular of machine tools, production machines, and robots.